An optical disc storage device can be either a device or system that is capable of retrieving information stored by an optical disc, or a device or system that is capable of both recording information to and retrieving information from an optical disc. Examples of optical disc storage devices that are capable of retrieving information from an optical disc include compact disc (CD) players, video laser disc (L) players and compact disc read-only-memory (CD-Rom) drives. Examples of optical disc storage devices that are capable of both recording information to an optical disc and retrieving information from an optical disc include recordable mini-disc (MD) players, magneto-optical (MO) disc drives and compact disc recordable (CD-R) drives.
Information is generally stored by an optical disc in the form of concentric or spiral tracks sometimes referred to as information tracks. In the case where information is already stored by an optical disc, the information tracks contain regions of optical contrast that represent the stored information. In the case of an unrecorded or blank optical disc containing per-formatted tracks for recording information, a track that will become an information track may or may not have regions of optical contrast. The area located between two information tracks on an optical disc is sometimes referred to as a non-information track.
When an optical storage device is in its normal mode of operation, i.e. retrieving information from or recording information to an optical disc, the storage device rotates the disc while using a light beam to retrieve information from or record information to the disc. As the optical disc rotates, the light beam radially traverses the disc. While the light beam traverses the optical disc, a tracking servo loop in the optical disc storage device keeps the beam of light centered on the information track, or the track that will become the information track in the case of recording information to a disc.
An optical disc tracking servo is a closed loop system that allows a light beam to remain centered on an optical disc information track during normal operation of an optical disc storage device. The tracking servo readjusts the radial position of the light beam by sensing when the light beam drifts off the center of the information track. The tracking servo senses when the light beam is not centered on an information track by measuring the intensity of light reflected by the surface of the optical disc.
Generally, the intensity of light reflected by the surface of an optical disc is the least when it is reflected by the center of an information track. Using this principle, a tracking servo generally senses the intensity of light reflected at one or both edges of an information track to detect when a light beam is drifting off center and to determine in which direction the light beam is drifting. Therefore, a tracking servo system that is in a closed loop mode of operation senses when the light beam floats off the center of the information track by detecting changes in the intensity of light reflected at one or both edges of an information track and moves the beam back into a position where the intensity of reflected light is optimal for center tracking.
In the case where a tracking servo measures the intensity of light reflected at both edges of an information track, the intensity of reflected light that is optimal for center tracking occurs when the intensity of light reflected at both edges of an information track is the same. The same principle holds true for both one and three beam optical disc storage devices. In the case where a tracking servo measures the intensity of light reflected at one edge of an information track, the intensity of reflected light that is optimal for center tracking is based on some calibrated value. The latter method is less favored due to difficulties associated with calibrating an appropriate centering value.
Optical disc storage devices are generally capable of performing various special operations to assist in positioning the light beam on the optical disc. These special functions are generally outside of the normal mode of operation of the storage device and include such operations as PAUSE, or still mode, and SEARCH, or seek mode. A PAUSE operation causes the light beam of a storage device to jump to an adjacent information track on the optical disc so that the most recently processed information is processed again by the storage device. A PAUSE operation gives the appearance of suspending normal operation of the storage device. The duration that operation of the storage device appears to be suspended depends on how many successive PAUSE operations are initiated such that the same information is repeatedly processed.
During a SEARCH operation, the optical storage device typically searches for a specific target track address on an optical disc. Achieving a search operation may require the light beam to radially cross several information tracks before the target address is found. Once the target address is found, the optical disc storage device can return to its normal mode of retrieving or recording information.
An optical disc storage devices typically performs a PAUSE or SEARCH operation, by disengaging the tracking servo loop for some period of time during the PAUSE or SEARCH operation. When the tracking servo loop is disengaged it is no longer closed and is referred to as being in an open loop mode, or simply that the loop is open. The tracking servo loop is disengaged (or open) during a PAUSE or SEARCH operation to allow the beam to freely move between information tracks. Keeping the tracking servo loop engaged (or closed) while attempting to move the beam between information tracks during a PAUSE or SEARCH operation would frustrate such attempted operations, because the tracking servo system would attempt to keep the beam centered on the information track. Therefore, optical storage devices generally keep the tracking servo loop open during some portion of a PAUSE or SEARCH operation.
As alluded to above, sending an overriding signal to a tracking servo to radially move a light beam across an optical disc while the tracking servo loop is closed would result in an inefficient or frustrated PAUSE or SEARCH operation. While more efficient than the latter, opening and closing a tracking servo loop while performing a PAUSE or SEARCH operation has a disadvantage of using up time that could otherwise be used by the optical disc storage device during its normal operating mode of retrieving or recording information.
One attempt to improve the efficiency of a seek operation proposes moving an optical head radially across a disc according to the characteristics of a seek profile signal that is supplied to the track positioning component of a tracking servo system independently from data read off the disc. Periodically sampled differences between the actual head position and the independently supplied seek profile signal are used to drive the track positioning component of the tracking servo system to adjust the head position so that the desired seek profile is maintained. Examples of seek operations that propose using an independently supplied seek profile signal and sampling can be seen in U.S. Pat. Nos. 4,980,876 and 5,210,726.
A disadvantage of the above proposed seek method, however, is that time is still expended sampling and adjusting differences between the actual head position and the independently supplied seek profile signal. Another disadvantage is that the above proposed seek method is susceptible to unpredicted noise and transients between sampling.